1. Field of the Invention
The present invention relates to a multiple access method for each terminal to effectively communicate to each other in a network using an orthogonal frequency division multiplex (OFDM) as a physical layer.
2. Description of the Related Art
In an OFDM-Code Division Multiple Access (OFDM-CDMA) technology, users share resources through a code division multiple access. The OFDM-CDMA technology is divided into a Multi Carrier-CDMA (MC-CDMA) technology and a MC-Direct Spread-CDMA (MC-DS-CDMA) technology according to a method of transmitting a user data symbol.
In the MC-CDMA technology, the CDMA and OFDM having an excellent frequency reuse efficiency and multiple access performance are combined to spread each user data symbol in a frequency domain through respectively different orthogonal codes. Accordingly, each data symbol is converted into a spread sequence as long as an orthogonal code length, and sequentially assigned not to overlap with sub-carriers having the number identical to that of a corresponding length and then transmitted.
The MC-DS-CDMA technology includes a related art MC-CDMA CDMA technology and a DS-CDMA technology, and also spread a spread sequence in a time domain. Since the spread sequence is not spread in a frequency domain, it is transmitted through the small number of sub-carriers in the MC-DS-CDMA technology.
In a case of a down link, since all user signals received in each user are influenced by an identical channel, the MC-CDMA technology is widely used to reduce interference between user signals and also multi-users. In a case of an up link, the MC-DS-CDMA technology having a low peak to average power ratio (PAPR) is widely used to reduce interference between adjacent channels.
The OFDM-CDMA technology spread information symbol of each user in a frequency and time domain. Additionally, the OFDM-CDMA technology has high efficiency because of a high processing and diversity gain when compared to the OFDMA technology using identical error correction symbols.
In the OFDM-CDMA technology, a Walsh Hadamard code, a PN code, a Gold code, an OVSF code, etc. are used as an orthogonal code. However, the performance of the OFDM-CDMA technology can not be maximized using those orthogonal codes. Since the orthogonal codes can not be processed using a parallel code perpendicular to each other, the diversity gain, which is very important in an OFDM system performance, can not be maximized.
Additionally, since a related art spread-based system transmits data using a single spread code, a resource application strategy such as power and channel coding, which can be flexibly applied according to a state of a radio channel, is based on a single spread sequence. Accordingly, the spread-based system can not be effectively adapted to a change of a channel.
Since the entire spread sequences includes a set of respectively independent sub-spread sequences, and each sub-spread sequence can utilize a respectively different resource application strategy, a higher spectrum usage efficiency can be achieved according to the state of the radio channel.
Additionally, an equalizer is required in a receiving unit to remove interference between multi-users in the OFDM-CDMA technology. Since complexity of the equalizer depends on a code length, it is difficult to embody an efficient receiving unit through a related art orthogonal code.
As described above, it is hard to optimize a system performance through an orthogonal code in the OFDM-CDMA technology, and also embody an effective structure of a receiving unit for removing interference between multi-users. Thus, a new orthogonal code is necessary to resolve these problems.